Potassium sulfate(SOP) is a specialty potash fertilizer produced in the amount of about 3 million tonnes per year. SOP is used on high value crops such as citrus and tobacco that are sensitive to the chloride ion in regular potash. It represents about 5% of the total potash fertilizer market. It is produced by combining potash with a source of sulfate. Some process plants use sulfate from sulfuric acid (the Mannheim processes of Belgium), others use magnesium sulfate as in Germany, some use langbeinite as in Carlsbad USA, while other process use sulfate brines such as at Great Salt Lake.
Some processes have used sodium sulfate such as plants at Searles Lake, Calif., started by Garrett Research and Development in the late 1930's and a plant operated by Potash Corporation of Saskatchewan in the 1980's. Process efficiencies were low. In another approach, a couple of processes developed by Potash Corporation (PCS) and Superfos of Denmark utilized a new process mechanism wherein anion exchange resin was employed with dilute sulfate brines. Scale up, process conditions and dilution due to resin absorption of water proved to be troublesome.
Despite the apparent simplicity of using sodium sulfate, which is available both as natural and synthetic material in North America and Europe, there is little use of this material as a direct feedstock. The major reason for this is the formation of an intermediate sodium potassium double salt termed glaserite when potash and sodium sulfate are reacted in proportions suitable for a high yield. The glaserite must be reacted with further potash to produce potash sulfate. Because of the complexity of the reaction there is a need for extensive recycle and evaporation to obtain significant yields. Evaporative loads of between 8 and 10 tonnes of water per tonne of product are needed which is very costly.
In an attempt to overcome the problems with the glaserite field, PCS and Superfos utilized anion exchange. Provided that the pH conditions and sulfate dilution was correct, the process worked within the limitations of conventional ion exchange devices. However, with the Superfos process, as levels of acidity rose in the crystallizer, formation of potassium bisulfate occurred which needed expensive potassium hydroxide to restore the system to conditions where potash would salt out potassium sulfate in the crystallizer.
In the PCS approach, only very dilute sulfate brines would fully displace chloride ion on the resin. As sulphate ion concentration rose, the resin efficiency dropped dramatically. This system worked satisfactorily only for dilute systems with a chloride ion bleed. For more concentrated solutions of sodium sulfate, such as one to two molar found to be desirable for most processes, process efficiencies become troublesome. With the low equivalency of anion resin (typically about 1.1 equivalents per litre in a working plant), the presence of chloride dropped resin capacity to the range of 0.5 equivalents per liter. The amount of resin needed for larger scale production grew significantly as did the amount of water absorbed by the resin into the process stream. The evaporative load becomes very high. With conventional fixed bed designs, dilution, washing losses and inefficiencies lead to high operating costs.